The present invention relates to a method for manufacturing a semiconductor device, and particularly to a method for manufacturing a semiconductor device wherein air gaps are formed between wirings formed on a semiconductor substrate.
There has heretofore been known a semiconductor device having air gaps provided between wirings with a view toward reducing capacitance between the wirings formed on a semiconductor substrate.
A method of manufacturing the semiconductor device provided with the air gaps includes a step for forming wirings on a first insulating film formed on a semiconductor substrate, so as to adjoin one another, a step for forming a second insulating film on the first insulating film by a plasma CVD method and covering the wirings with the second insulating film such that the air gaps are formed between the adjacent wirings, and a step for forming a third insulating film high in moisture resistance on the second insulating film. Such a configuration has been disclosed in, for example, a patent document 1 (Japanese Unexamined Patent Publication No. Hei 8(1996)-213392).
The patent document 1 discloses that as shown in FIG. 1 and paragraphs 0011 to 0014, top layer wirings 203 are formed on an insulating film 202 formed on a semiconductor substrate 201 so as to adjoin one another, a silicon oxide film 204 is formed on the insulating film 202 by a plasma CVD method and covers the top layer wirings 203 in such a manner that cavity portions 207 are formed between the top layer wirings 203, and a silicon nitride film 205 corresponding to a final protective film is further formed on the silicon oxide film 204.
Since, however, the insulating film for forming the air gaps is formed by the plasma CVD method low in steplike coating property in the conventional configuration referred to above, the flatness of the surface of the insulating film is not satisfactory. Therefore, there is a possibility that the coating property of the insulating film high in moisture resistance, which is formed on the insulating film, will be degraded. Thus, there was a possibility that the moisture resistance of the semiconductor device would be degraded.